Traditionally, polymers have been used in applications such as adhesives, matrices for composites and dielectrics for electrical circuits. For example, epoxies were first introduced in the 1950's and are still used in matrices for composites and in adhesives. Similarly, polyimides were developed in the 1960's and are used in high temperature applications in the range of about 150.degree. C. to about 340.degree. C. Typically, epoxies have a thermal stability of up to 160.degree. C. and therefore are not used in high temperature applications. Moreover, the epoxies and polyimides have a tendency to pick up moisture, have dielectric constants in the range of about 5.0 to about 3.3 and have poor corrosion resistance.
Polymers such as polyimides, polybenzimidazoles and polyquinoxalines have been the polymers of choice for high temperature applications. However, these polymers have drawbacks. For example, these polymers can be difficult to process because they require high curing temperatures and venting under a vacuum may be necessary to control the release of volatiles during the curing process. In addition, such polymers typically must be applied to a substrate after the substrate has been heated and, most preferably, after the substrate has been anodized. Moreover, some of these polymers may require additives that inhibit corrosion and a diluting solvent. In spite of the serious drawbacks associated with these polymers, no viable alternatives have emerged despite intense research.
While crosslinked copolyesters offer certain advantages, including the ability to withstand continuous use temperatures in air of 350.degree. C., inertness to moisture, high strength and modulus values, outstanding dimensional stability, and low dielectric constants, crosslinked polyesters were not considered for use in adhesives, composites, rigid foams, protective coatings and as dielectrics because of various preconceptions. For example, one reason why polyesters have not been evaluated as potential adhesives is the observation that good adhesion between polymers can only be achieved if the polymer chains penetrate up to several hundred angstroms across the interface between two polymers. Presumably, the rod-like nature of many aromatic and aromatic-aliphatic polyesters would reduce the potential for chain entanglement, and thus not be conducive to good adhesion between two polyesters. This is particularly true in the case of liquid crystalline copolyesters. Moreover, the penetration of polymer chains in the case of crosslinked polyesters would be even less, as crosslinking creates a more structured three dimensional polymer, and crosslinked polymers are generally considered to be inert.
However, it has been unexpectedly found that good adhesion may be obtained between two crosslinked copolyesters because the copolyesters undergo interchain transesterification reactions at the interface between the two copolyesters.
The suitability of crosslinked copolyesters as substitutes for other polymers in composites, dielectrics and protective coatings was also not investigated, as no way to produce high density crosslinked copolyesters in situ was apparent in light of the known tendency of copolyesters to release volatiles upon crosslinking or curing. The released volatiles usually resulted in a foam or other low density composition that was not suitable for many applications.